Let us give you - developer some hints in the report creation.

• C/SIDE database - use C/ODBC to create the connection for your Crystal Report. You have to make all the links inside the report. The drawback of this approach - report becomes inflexible if you would like to change database mapping (re-link from work to historical tables for example) - it is probably easier to redo report from scratch. You can use something like Crystal Query - which is still very restricted in its functionality.
• Microsoft SQL Server - you are better positioned to create Crystal Reports. You can deploy MS SQL Server views. View will allow you to unionize work and historical info, pull the data from several databases to name a few nice features. Also the option to use stored procedures (or stored procs) enables you to create temp tables for data manipulation before sending the final result set. You can also have parameters (name of the company/database, dates restrictions) - this gives you additional advantage versus SQL View
• Reports on geterogeneus databases - imagine creation of Linked Server in your Microsoft SQL Server Enterprise Manager. If Navision sits in MS SQL - you create linked server to the third party database (Oracle, DB2, Unidata, Ingress, Sybase, Pervasive, Ctree, MS Access). If you are on C/SIDE DB - you create linked server to it via C/ODBC, then you create additional linked servers to your legacy DBs. Now you are ready to create geterogeneus SQL View to link all your databases and have the data shown in one Crystal Report. You should probably familiarize yourself with OPENROWSET statement in Transaction SQL

Good luck in reports design and if you have issues or concerns